1. Field of the Invention
The present invention relates to an etch of polysilicon film, and more specifically, to a method for avoiding over etch abnormal of the polysilicon film.
2. Description of the Prior Art
The gate performance of semiconductor process is effected greatly by the grain structure of polysilicon film. The grain structure change due to a different process condition will induce electricity abnormal of devices and reduce the products yield. In depositing column structure polysilicon film or random structure polysilicon film, the depositing film surface has a different growth rate due to grain orientation to cause grain structure surface forming a deeply void. The void will lead to polysilicon film over etch abnormal in the proceeding etch process, therefore, affect the electricity of the devices enormously, thereby scraping the products.
Above mentioned, the gate performance of semiconductor process is effected greatly by the grain structure of polysilicon film. The forming structure of the polysilicon film includes column structure and random structure. Generally speaking, the random structure has more superior protection ability than column structure in Boron penetration. Therefore, in the advanced process, the random structure polysilicon film is selected to keep more stable electricity.
In depositing the polysilicon film step, the polysilicon film form a rough and uneven surface due to the different orientation grain growth rate. The height difference of the polysilicon film surface thickness reaches above 10% total thickness of the polysilicon film. Moreover, in the proceeding etch process of the polysilicon film is used by over etch method to remove the expected removal polysilicon film area completely to expose a silicon oxide layer, thereby the silicon oxide layer at the top of the substrate can protect the substrate under the silicon oxide layer.
FIG. 1 to FIG. 4 are process of forming void in substrate in the step of etching polysilicon film according to the prior art. The substrate will produce void due to the forming rough and uneven surface polysilicon film in the proceeding etch process of the polysilicon film. Referring to FIG. 1, the patterned photoresist 16 is formed on the polysilicon layer 14, wherein the polysilicon layer 14 is formed on silicon oxide layer 12 at the top of the substrate 10. Subsequently, an uncovered polysilicon layer 14 is removed gradually in the etching step to form a polysilicon layer 18. In the etching step, the silicon oxide layer 12 at the top of the substrate 10 can protect the substrate 10 from etching, illustrated diagrammatically in FIG. 2. Next, due to the rough and uneven surface polysilicon layer 14 and too thin silicon oxide layer 12, the silicon oxide layer 12 will be etched to form a polysilicon layer 22 and a local void 20, illustrated diagrammatically in FIG. 3. Successively, in the remaining polysilicon layer 22 proceeding etching, because the substrate 10 has the same silicon material with the remaining polysilicon layer 22, the substrate 10 will be etched quickly to form a local void 24, illustrated diagrammatically in FIG. 4.
For the sub-micron semiconductor devices, if the height difference of the polysilicon film surface thickness is too large, the silicon oxide layer under the thickness thinner polysilicon film will be etched abnormally to form a void, as mentioned above. Moreover, reduction the over etch extent may be made the polysilicon film removed incompletely, thereby the process window can not be controlled by the over etch method.
Therefore, in view of the prior art drawback, considering more thinner silicon oxide thickness and polysilicon film over etch margin, a method of avoiding the polysilicon film surface void must be developed to avoid silicon oxide layer etched abnormally to form a void.